1. Field of the Invention
The present invention relates to a semiconductor acceleration sensor which utilizes the piezoresistive effect of piezoresistors formed on a semiconductor substrate.
2. Description of the Background Art
Recently, much attention has been attracted to a small size semiconductor acceleration sensor in which the detection of an acceleration is achieved by detecting a small capacity change or the change of the resistivity caused by the piezoresistive effect of the piezoresistors formed on a semiconductor substrate.
Such a semiconductor acceleration sensor is manufactured by using a circuit integration technique, so that it can be manufactured in the form of a compact chip element of approximately 1 mm.times.1 mm size, with a vibrator portion having a length approximately equal to 100 .mu.m and a thickness approximately equal to 1 .mu.m. In addition, it is also possible to form this semiconductor acceleration sensor on a substrate commonly shared with the other circuit elements.
Such a semiconductor acceleration sensor has a wide range of applications, among which a typical example is an acceleration sensor for an automobile.
An example of a conventional semiconductor acceleration sensor for an automobile is shown in FIG. 1 and FIG. 2.
In this semiconductor acceleration sensor of FIG. 1 and FIG. 2, there is provided a cantilever beam 2 having a weight portion 3 on its free end and a piezoresistor 5 in the vicinity of its fixed end. This cantilever beam 2 is arranged in a space 10 formed on a frame 4 such that the weight portion can freely vibrate between a lower stopper 7 for limiting a downward displacement of the weight portion 3 and an upper stopper 8 for limiting an upward displacement of the weight portion 3, with the fixed end of the cantilever beam 2 fixedly attached to the frame 4. In addition, another piezoresistor 6 which is identical to the piezoresistor 5 is formed on the frame 4, so as to form a bridge circuit.
This semiconductor acceleration sensor is to be positioned to have the acceleration applied in a direction perpendicular to the sensor chip, such that the displacement of the cantilever beam 2 caused by the acceleration can be detected as a change in the resistivity of the piezoresistor 5. Here, the possible damaging of the cantilever beam 2 due to the excessive acceleration is prevented by limiting the displacement of the weight portion 3 with the lower and upper stoppers 7 and 8.
Such a semiconductor acceleration sensor can be manufactured by applying an IC manufacturing process to a silicon substrate 1.
However, in manufacturing such a conventional semiconductor acceleration sensor by applying an etching process on a semiconductor substrate from a wafer surface so as to form the cantilever beam 2 and the weight portion 3, there has been a problem that, as shown in FIG. 3, it has been quite difficult to place a center of mass 31 of the weight portion 3 at the same vertical level as the cantilever beam 2, while on the other hand the presence of a vertical deviation .delta.L between the center of mass 31 of the weight portion 3 and the cantilever beam 2 can produce an undesirable cross axial sensitivity in the semiconductor acceleration sensor.
Namely, when the distance between the center of mass 31 of the weight portion 3 and the fixed end of the cantilever beam 2 is L and there is a vertical deviation .delta.L between the center of mass of the weight portion 3 and the cantilever beam 2, the semiconductor acceleration has a cross axial sensitivity S.sub.cross =.delta.L/L.
To cope with this problem, there has been a proposal for additionally providing a metallic weight 41 on top of the weight portion 3 as shown in FIG. 4, so as to bring the center of mass up to the same vertical level as the cantilever beam 2. However, such a configuration will complicate the manufacturing process of the semiconductor acceleration sensor. Moreover, an addition of a member made of a material different from the rest of the semiconductor acceleration sensor presents a very serious problem concerning thermal stress.
On the other hand, there has been another proposal for shaping the semiconductor acceleration sensor as shown in FIG. 5 by applying an etching process on a semiconductor substrate from both sides of a wafer, such that the center of mass of the weight portion can be placed at the same vertical level as the cantilever beam 2. However, such a configuration will also complicate the manufacturing process of the semiconductor acceleration sensor, as the deep etching has to be applied to both sides of the wafer.
Here, for the purpose of manufacturing a silicon cantilever beam with a highly accurate thickness, the etching process may be achieved by the electrochemical etching stop method in which an electrochemical etching is carried out in a solution of a strong alkali such as potassium hydroxide (KOH) or by the etching stop method using a high concentration p-type diffusion layer. However, in such cases, the formation of the epitaxial layer or a high concentration p-type diffusion layer required by these etching process will be quite difficult.
Moreover, with a configuration shown in FIG. 5, it becomes quite difficult to carry out a photolithography for forming the piezoresistors and metallic wirings.